A variety of techniques are known for pump control and power management in petroleum well pumping systems. Typically, land-based pumping systems comprise a beam pump, an electric motor, a plunger and a barrel. Such beam pump units are known in the petroleum industry as xe2x80x9cpump jacksxe2x80x9d or xe2x80x9crod pumps.xe2x80x9d Generally, these pumps are driven by AC electric motors.
A variety of techniques have been devised to regulate the speed of the beam pump unit to match the fluid accumulation in the well. For example, some pump speed control systems employ what is known as xe2x80x98damper regulationxe2x80x99 or xe2x80x98throttling.xe2x80x99 In damper regulation, the motor runs at a fixed speed, but works against a restriction. This results in relatively high energy consumption. In such constant speed systems, high torques are encountered on startup, the upstroke and the downstroke of a pump stroke cycle.
Alternatively, magnetic couplings, known as eddy current drives, are employed to vary the speed of the beam pump unit to match the oil well""s production capabilities. Eddy currents xe2x80x9cslipxe2x80x9d a magnetic clutch to vary the speed of the pump. Clutch slip is inefficient, generates excessive heat in the clutch, and results in loss of energy. Further, eddy current drives can only slow, and not increase the speed of the beam pump.
Another issue in beam pump motor management is xe2x80x9cpump-off.xe2x80x9d While under extraction, the oil well can eventually pump off causing the plunger to xe2x80x9cpoundxe2x80x9d the fluid in the barrel. To eliminate pounding, simple pump off controllers are installed to stop and restart the pump after a predetermined amount of time. In general, pump off controllers prevent damage to the system, but do not decrease mechanical loading. Consequently, they offer insignificant savings in energy usage.
Many attempts have been made to use variable speed drives to control the motor that drives the beam pump units. Such attempts have met with limited success. Variable drives, and the loads they are applied to, can generally be divided into two broad categories: xe2x80x9cconstant torquexe2x80x9d and xe2x80x9cvariable torque.xe2x80x9d
Even though called xe2x80x9cconstant torque,xe2x80x9d the term xe2x80x9cconstant torquexe2x80x9d is not literally accurate. The torque actually required can vary significantly. For example, xe2x80x9cconstant torquexe2x80x9d applications frequently experience overload conditions with high torque demands. As a result, xe2x80x9cconstant torquexe2x80x9d variable drives typically have a high overload rating (for example, 150% for 60 seconds) to handle the high peak torque demands. To handle these high torque excursions, excess horsepower is required. Consequently, additional energy is consumed during peak torque periods. This excess energy is wasted when torque demands are low. Thus, the beam pump unit is energy inefficient.
In another type of drive, known as a xe2x80x9cconstant speedxe2x80x9d variable drive, the driving motor is driven at a desired constant speed. As with constant torque applications, constant speed drives have irregular energy usage characteristics. During a typical cycle of a cyclic load including startup, as well as parts of the upstroke and downstroke, constant speed drives require significant energy. They are particularly energy inefficient when in xe2x80x9cregenerationxe2x80x9d mode.
Regeneration is caused when the load xe2x80x9cover speedsxe2x80x9d the electric motor. This transforms the motor into a generator that generates excessive energy that returns to the variable drive. Generally, variable drives are not designed to absorb electrical energy from the motor. To handle the regenerated power, power dissipation devices such as resistors are employed. Use of resistors to consume regenerative power is very energy inefficient. There are, however, what are known as regenerative variable drives. Such drives are, however, generally expensive and do not reduce energy consumption.
Consequently, there in a need for a system and method for an energy efficient and cost effective way to control a motor driving a variable cyclic load.
A system and method for controlling the speed of an electric motor that drives a cyclic load is provided. In one embodiment, the system includes a signal conversion circuit, a signal measurement circuit, a control circuit, a signal inversion circuit and, optionally, a user interface circuit. The signal conversion circuit converts an AC electric signal to a DC electric signal. The signal measurement circuit periodically samples the DC electric signal to derive a DC electric characteristic signal from the DC electric signal. The control circuit receives the DC electric characteristic signal from the signal measurement circuit and derives and stores a DC current signal from the set of DC electric characteristic signals. The control circuit is responsive to the DC electric characteristic signal and a set of provided or derived operational parameters which may include, for example, a voltage-frequency profile and a set point parameter to generate a set of control signals. The signal inversion circuit, responsive to the set of control signals, inverts the DC electric signal and generates a set of drive signals to modify the speed of the motor through changes in supplied frequency or voltage. By modifying the motor speed, the system and method controls and regulates the motor attributes of speed and torque to selectively operate the motor either in a regulated xe2x80x9cconstant torquexe2x80x9d mode or in a regulated xe2x80x9cconstant speedxe2x80x9d mode.
In the regulated xe2x80x9cconstant torquexe2x80x9d mode, in response to a set point parameter provided as a torque set point, the control circuit derives a set of torque reference signals. The control circuit compares the received set of DC electric characteristic signals and the stored DC current signal with the set of torque reference signals to generate a set of control signals. The control signals are representative of a determined selective adjustment to the frequency and voltage of the set of the drive signals. The signal inversion circuit generates a corresponding set of variable frequency and variable voltage drive signals and applies the set of variable frequency and variable voltage drive signals to selectively drive the motor at increased or decreased speeds by incrementing or decrementing the frequency of the set of drive signals. In alternative modes, the voltage of the drive signals may be modified.
In the regulated xe2x80x9cconstant speedxe2x80x9d mode, in response to the set point parameter provided as a set point target torque range for a selected motor speed, the control circuit derives a set of speed reference signals. The control circuit compares the received set of DC electric characteristic signals and the stored DC current signal with the set of speed reference signals to generate the set of control signals. The control signals are representative of the selective adjustment to the frequency and voltage of the set of the drive signals. The signal inversion circuit generates a set of variable frequency and variable voltage drive signals and applies the set of variable frequency and variable voltage drive signals to selectively drive the motor at increased or decreased speeds by incrementing or decrementing the frequency of the set of drive signals according to the set point target torque range for the selected motor speed.
The optional user interface circuit is a supplied method to provide the control circuit with the set of operational parameters such as a voltage-frequency profile, and set point parameter appropriate for either the regulated xe2x80x9cconstant torquexe2x80x9d or regulated xe2x80x9cconstant speedxe2x80x9d mode. The set point parameter represents a motor control characteristic. The motor control characteristic may be provided to the system or derived from a motor load profile that typifies a set of characteristics of the motor under control. The motor load profile may be determined in the system by driving the motor through a complete cycle of the cyclic load.
The motor load profile may optionally be derived using a load sensor and a speed sensor. The motor may be operated at a selected motor torque to obtain a variable motor speed characteristic in the unregulated xe2x80x9cconstant torquexe2x80x9d domain. The motor may also be operated at a selected motor speed to obtain a variable motor torque characteristic in the unregulated xe2x80x9cconstant speedxe2x80x9d domain. The variable motor control characteristics are preferably derived as the motor runs through the cycle of the cyclic load and the load sensor and speed sensor generate data representative of the characteristics of the unregulated motor under loading conditions.